(CNN) – Former Sen. Scott Brown on Monday stepped into the attack dog role for Donald Trump, saying his former Senate opponent Elizabeth Warren should "take a DNA test" if she wants to prove her Native American heritage.

"She's not Native American, she's not 1/32nd, she has no Native American background, except for what her family told her," Brown told reporters on a Trump campaign call happening hours after Hillary Clinton and Warren appeared at an event together. "The easy answer, as you all know, is that Harvard and Penn can release those records, she can authorize the release of those records, she can take a DNA test, she can release the records herself. There's never been any effort," Brown said.

CNN has reached out to Warren and the Clinton campaign for comment.

Warren and Clinton appeared at a campaign event together earlier in the day, a large part of which was spent slamming Trump, the presumptive Republican nominee.

Warren has previously said her mother was "part Cherokee and part Delaware." Brown called the Massachusetts Democrat's claimed Native American heritage "a reverse form of racism," showing "very serious character flaws."

"Secretary Clinton is considering making someone the vice president who has very serious character flaws when it comes to honesty and credibility and dealing with her heritage, and what did that do? That took away somebody who truly was a Native American and gave that opportunity to somebody who is not, and that's just not right. It's a reverse form of racism, quite frankly."

The issue has come up before, in the the 2012 Massachusetts Senate race. Brown and supporters frequently brought it up on the campaign trail. Warren, a first-time candidate, defeated Brown, who later moved to New Hampshire and ran unsuccessfully for Senate from the Granite State.

Brown has since become a prominent surrogate for Trump. On Monday he also critiqued Warren's willingness to appear alongside Clinton, whom he dubbed "Queen of Wall Street," calling her support of the presumptive Democratic nominee "hypocritical."

Sources tell CNN Warren is among those under consideration for Clinton's vice presidential pick. Asked whether he hopes she makes the cut, Brown said Clinton has "much better choices," pointing to Virginia Sen. Tim Kaine and New Jersey Sen. Cory Booker, whom he described as "more vetted, more logical choices."

As vice presidential speculation fuels for Brown himself, he would not answer whether he is being vetted by the Trump campaign.

"He will release those names when he feels it's appropriate," Brown said, directing inquiries to the Trump campaign.